The present invention relates in general to an image display device of the type which utilizes the emission of electrons in a vacuum which is created between a face substrate and a back substrate; and, more particularly, the invention relates to a secondary electron emission suppression structure which can reduce the reentry of secondary electrons that are generated when electron beams impinge on an anode.
As a display device which exhibits high brightness and high definition, color cathode ray tubes have been widely used conventionally. However, along with the recent demand for images of higher quality in information processing equipment or television broadcasting, the demand for planar displays (panel displays), which are light in weight and require a small space, while exhibiting high brightness and high definition, has been increasing.
As typical examples, liquid crystal display devices, plasma display devices and the like have been put into practice. More particularly, as display devices which can produce a higher brightness, various kinds of panel-type display devices, including a display device which utilizes an emission of electrons from electron sources into a vacuum (hereinafter referred to as “an electron emission type display device” or “a field emission type display device”, and also as a “FED”) and an organic EL display, which is characterized by low power consumption, have been commercialized.
Among such panel type display devices, such as the above-mentioned field emission type display device, particularly, a display device having an electron emission structure, which was invented by C. A. Spindt et al, a display device having an electron emission structure of a metal-insulator-metal (MIM) type, a display device having an electron emission structure which utilizes an electron emission phenomenon based on a quantum theory tunneling effect (also referred to as a “surface conduction type electron source”), and a display device which utilizes an electron emission phenomenon having a diamond film, a graphite film or carbon nanotubes have been known.
The field emission type image display device includes a back substrate, on which cathode lines are formed which have field-emission-type electron sources and control electrodes on an inner surface thereof, and a face substrate, on which anodes and phosphors are formed on an inner surface thereof which faces the back substrate in an opposed manner. These substrates are laminated to each other with a sealing frame interposed between the inner peripheries thereof, and the inside defined by both substrates and the sealing frame is evacuated to create a vacuum therein. The above-mentioned control electrodes are arranged to cross the cathode lines by way of an insulation layer or an insulation gap.
Further, in the above-mentioned control electrodes, a single hole or a plurality of holes, which allow electrons emitted from the electron sources formed on the cathode lines to pass therethrough, are formed for every pixel. Further, for maintaining a given distance between the back substrate and the face substrate, distance holding members may be provided between the back substrate and the face substrate. The distance holding members for maintaining a given distance between the back substrate and the face substrate are, for example, formed of a thin plate made of glass or ceramics and are arranged in an erect manner at positions which avoid the pixels.
FIG. 7 is an enlarged cross-sectional view of the vicinity of one pixel for illustrating the basic structure of an FED. In FIG. 7, the FED includes a back panel PN1, on which cathode lines CL are formed having cathodes K position thereon which constitute field-emission-type electron sources and on which control electrodes G are formed on an inner surface of a back substrate SUB1, and a face panel PN2, on which phosphors PHS, a black matrix BM and an anode ADE are formed on an inner surface of a face substrate SUB2, which faces the back panel PN1 in an opposed manner and which has a light transmitting property. The back panel PN1 and the face panel PN2 are laminated to each other with a sealing frame interposed between the inner peripheries of both of the panels PN1, PN2, thus forming a vacuum vessel (envelope), the inside of the vacuum vessel being evacuated to create a vacuum therein. Here, with respect to this type of technology, for example, examples of such display devices are described in Japanese Laid-open Patent Publication Hei10(1998)-134701 and Japanese Laid-open Patent Publication 2000-306508.
In an FED having such a constitution, the control electrodes G, which form electron passing holes EHL, are arranged between the cathodes K, formed on the cathode lines CL provided on the back substrate SUB1, and the anode ADE formed on the face substrate SUB2. By applying given potential difference between the control electrodes G and the cathode lines CL, electrons E are induced from the cathodes K, and the electrons E are allowed to pass through the electron passing holes EHL formed in the control electrodes G and to impinge on the phosphors PHS formed on the anode ADE, thus enabling an image display.
Further, the FED having such a constitution is configured such that the face-to-face distance between the anode ADE and the cathode lines CL is set to approximately several mm. Here, the FED is driven by applying a high voltage, which constitutes an anode voltage Eb (Eb=13±3 kV), to the anode ADE to allow the phosphors PHS to efficiently emit light, by applying a voltage which constitutes a control voltage Eg (Eg=1 kV or below 1 kV) to the control electrodes G, and by applying a cathode voltage Ek (Ek=several 100V) to the cathodes K.
Accordingly, in the FED, the anode voltage Eb is extremely high compared to the respective electrode voltages, and, hence, a steep potential gradient is formed between these voltages. Accordingly, as shown in FIG. 8, secondary electrons e2, including reflecting electrons, which are generated when primary electrons e1 which form the electron beams impinge on the anode ADE and reenter the anode ADE due to an electric field, excite the neighboring phosphors and cause the phosphors to emit light. Accordingly, in a peripheral portion of the display image which actually emits light, halation attributed to the reentry of the secondary electrons e2 is generated and this phenomenon has been the main cause of the lowering of the contrast.
As a means to overcome such a drawback, there is an image display device in which an aluminum layer is formed over the phosphors PHS, which are formed on the anode ADE in a state in which the film thickness of the aluminum layer is adjusted so as to suppress the reentry efficiency of the secondary electrons into the aluminum layer to 30% or less. Accordingly, the halation can be reduced within a range in which the brightness is slightly lowered, whereby the contrast is enhanced. Here, with respect to this type of technology, Japanese Laid-open Patent Publication Hei5(1993)-314932 can be cited as an example.
Further, as another means to overcome such a drawback, there is a field emission type display device in which a first conductive layer (an aluminum layer), which constitutes a light reflection layer, and a second conductive layer (a carbon layer), which controls the scattering of the primary electrons, are sequentially formed over the phosphors which are is formed on the anode ADE; and, hence, these layers can simultaneously function as a metal back layer and provide a scattering suppressing function, whereby the contrast and the color purity can be enhanced. Here, with respect to this type of technology, Japanese Laid-open Patent Publication Hei10(1998)-321169 can be cited as an example.